1. Field of the Invention
The present invention generally relates to a semiconductor sensor production method and a semiconductor sensor produced by using such a method, and more particularly relates to a method of producing a semiconductor sensor using piezoresistors, such as a semiconductor acceleration sensor or a semiconductor angular rate sensor, and a semiconductor sensor produced by using such a method.
A semiconductor sensor is used, for example, to measure acceleration applied to a moving automobile in the direction of travel or in the lateral direction; or to measure camcorder shake.
2. Description of the Related Art
A semiconductor multi-axis acceleration sensor having sensitivity to acceleration in multiple directions is known as an example of a semiconductor acceleration sensor using piezoresistors (refer to patent document 1, for example).
FIG. 4A is a plan view of an exemplary conventional semiconductor sensor. FIG. 4B is a cross-sectional view of the exemplary conventional semiconductor sensor taken along line C-C shown in FIG. 4A. In FIG. 4B, 2 indicates a support part segment, 4 indicates a proof mass part segment, 6 indicates a beam part segment, 7 indicates an opening segment where substrate material is removed in the thickness direction, 8 indicates a front side silicon layer of an SOI substrate, 9 indicates a back side silicon layer of the SOI substrate, and 10 indicates a buried oxide film formed between the front side silicon layer 8 and the back side silicon layer 9. In FIG. 4A, illustration of a passivation film is omitted, and, instead, wiring patterns 23 and piezoresistors 24 are illustrated. In FIG. 4B, illustration of the wiring patterns 23 is omitted.
The exemplary conventional semiconductor sensor includes a proof mass part 20 which is bent according to acceleration and a frame-shaped support part 18 formed around the proof mass part 20. The opening segments 7 are positioned between the support part 18 and the proof mass part 20. Beam parts 22 are formed between the proof mass part 20 and the support part 18. One end of each beam part 22 is connected to the proof mass part 20 and the other end of the beam part 22 is connected to the support part 18.
The support part 18 is made of the front side silicon layer 8, the back side silicon layer 9, the buried oxide film 10, an interlayer insulation film 26 formed on the front side silicon layer 8, and the passivation film 27 formed on the interlayer insulation film 26. The front side silicon layer 8, the back side silicon layer 9, and the buried oxide film 10 constitute the SOI substrate. The wiring patterns 23 and electrode pads 25 are formed on the support part segment 2 of the interlayer insulation film 26. Parts of the passivation film 27 corresponding to the electrode pads 25 are removed, and therefore the electrode pads 25 are exposed on the front side of the semiconductor sensor.
The proof mass part 20 is made of the front side silicon layer 8, the back side silicon layer 9, the buried oxide film 10, the interlayer insulation film 26, and the passivation film 27. The front side silicon layer 8, the back side silicon layer 9, and the buried oxide film 10 make up the SOI substrate, which also constitutes a portion of the support part 18.
The thickness of the back side silicon layer 9 of the proof mass part 20 is less than the thickness of the back side silicon layer 9 of the support part 18. A base 16 is bonded by anodic bonding to the back side of the back side silicon layer 9 of the support part 18. A gap is provided between the proof mass part 20 and the base 16, making room for the proof mass part 20 to move.
The beam parts 22 are made of the front side silicon layer 8, which also constitutes portions of the proof mass part 20 and the support part 18, the interlayer insulation film 26, and the passivation film 27. The piezoresistors 24 are formed on the front side silicon layer 8 of the beam parts 22 by using a diffusion method used in semiconductor manufacturing. The wiring patterns 23 formed on the beam part segments 6 of the interlayer insulation film 26 are electrically connected through “through holes” (not shown) formed in the interlayer insulation film 26 to the piezoresistors 24.
In this conventional semiconductor sensor, the direction of the SOI substrate thickness is called the Z axis direction, the direction which is parallel to a plane orthogonal to the Z axis and parallel to a side of the support part 18 is called the X axis direction, and the direction which is parallel to the plane and perpendicular to the X direction is called the Y axis direction. The proof mass part 20 is suspended from the support part 18 by a pair of beam parts 22 formed in the X axis direction and a pair of beam parts 22 formed in the Y axis direction. Four piezoresistors 24 are formed on the pair of beam parts 22 formed in the X axis direction, two on each beam part 22. Each two piezoresistors are electrically connected by a wiring pattern 23 so as to form a bridge circuit for detecting the displacement in the X axis direction. Eight piezoresistors 22 are formed on the pair of beam parts 22 formed in the Y axis direction, four on each beam part 22. Each two piezoresistors 22 are electrically connected by a wiring pattern 23 so as to form two sets of bridge circuits for detecting the displacement in the Y axis and Z axis directions.
With a configuration as described above, when an external force (acceleration) containing a component in the X axis, Y axis, or Z axis direction is applied to the semiconductor sensor, the proof mass part 20 is bent due to inertia in relation to the support part 18. As a result, the beam parts 22 bend and the resistances of the piezoresistors 24 formed on the beam parts 22 change. Acceleration applied to the semiconductor sensor in the X axis, Y axis, and Z axis directions can be determined by detecting the changes in the resistances of the piezoresistors 24.
FIGS. 5A through 5F are cross-sectional views of the conventional semiconductor sensor shown in FIG. 4A, which cross-sectional views are used to describe a conventional semiconductor sensor production method. Steps (a) through (f) described below correspond to FIGS. 5A through 5F. In FIGS. 5A through 5F, illustration of the piezoresistors, metal wiring patterns, and passivation film is omitted.
(a) A silicon oxide film is formed on the entire back side of the back side silicon layer 9 of the SOI substrate by using a plasma CVD method. A first etching mask layer 12 made of silicon oxide is formed on the support part segment 2 by selectively removing the silicon oxide film using a photolithography or etching technique.
(b) A second etching mask layer 14 is formed on the first etching mask layer 12 and on the proof mass part segment 4 of the back side silicon layer 9 by using a photolithography technique. Dry etching is performed from the back side of the SOI substrate by using the second etching mask layer 14 as a mask. This dry etching is performed until the buried oxide film 10 in the segments other than the support part segment 2 and the proof mass part segment 4 is exposed on the back side of the SOI substrate.
(c) The second etching mask layer 14 is removed. The thickness of the proof mass part segment 4 of the back side silicon layer 9 is reduced by etching a portion of the back side silicon layer 9 from the back side of the SOI substrate by using the first etching mask layer 12 as a mask.
(d) The first etching mask layer 12 is removed by using a buffered hydrofluoric acid solution. In this step, the buried oxide film 10 exposed on the back side of the SOI substrate in the segments other than the support part segment 2 and the proof mass part segment 4 is also removed.
(e) A third etching mask layer (not shown) having openings in the opening segments 7 (segments other than the support part segment 2, the proof mass part segment 4, and the beam part segments 6) is formed on the front side silicon layer 8 of the SOI substrate by using a photolithography technique. The front side silicon layer 8 is selectively removed by performing dry etching using the third etching mask layer as a mask. As a result, the support part 18, the proof mass part 20, and the beam parts 22 are formed. After the dry etching is completed, the third etching mask layer is removed by oxygen plasma ashing.
(f) A glass substrate as the base 16 is bonded by anodic bonding to the support part segment 2 of the back side silicon layer 9.
Since the proof mass part segment 2 of the back side silicon layer 9 has been etched in step (c), there is no need to form a concave portion on the glass substrate used as the base 16 in order to form a gap between the proof mass part 20 and the base 16. Therefore, a flat glass substrate can be used as the base 16.
[Patent document 1] Japanese Patent Application Publication No. 2005-49130
However, in the exemplary conventional semiconductor sensor production method described above with reference to FIGS. 5A through 5F, edges 9a of the support part segment 2 of the back side silicon layer 9 may be etched as shown in FIG. 6 during the dry etching processes in steps (b) and (c). As a result, a gap may be formed between the back side silicon layer 9 and the first etching mask layer 12.
If the wet etching process in step (d) is performed with such a gap, air bubbles may attach to the edges 9a of the back side silicon layer 9. These air bubbles may prevent complete removal of the first etching mask layer 12 and a portion of the first etching mask layer 12 may remain as an etching residue on the support part segment 2 of the back side silicon layer 9. Such an etching residue makes the back side of the support part 18 uneven, making it difficult to securely bond the base 16 and thereby lowering the yield.
Although dry etching may be used instead of wet etching to remove the first etching mask layer 12, since the buried oxide film 10 of the SOI substrate is positioned far from the back side of the support part 18, the buried oxide film 10 exposed on the back side of the SOI substrate and the first etching mask layer 12 cannot be removed at the same time with dry etching.
Also, since the first etching mask layer 12 and the second etching mask layer 14 have the same size as shown in FIG. 5B, if misalignment occurs in the photolithography process and the first etching mask layer 12 and the second etching mask layer 14 are misaligned, and if the second etching mask layer 14 is removed in such a condition, a portion of the first etching mask layer 12 may protrude as shown in FIG. 7 from the support part segment 2 of the back side silicon layer 9. If wet etching is performed with such a protrusion 9a of the first etching mask layer 12 as shown in FIG. 7, air bubbles may attach to the protrusion 9a. These air bubbles may prevent complete removal of the first etching mask layer 12 and a portion of the first etching mask layer 12 may remain as an etching residue on the back side of the support part 18. Such an etching residue makes it difficult to securely bond the base 16, thereby lowering the yield in semiconductor sensor manufacturing.